Brown's GCE Chemistry
Advanced A Level Organic Chemistry
A Level Revision Notes PART 10
Summary of organic reaction mechanisms
A mechanistic introduction to organic chemistry and
explanations of different types of organic reactions
free radical thermal cracking
What is the free radical mechanism for the cracking of
alkanes? what free radicals are formed in cracking reactions of alkanes? what is
the reaction mechanism for converting alkanes to alkenes? What is the importance
of cracking to the petrochemical industry? How is cracking used in the oil
industry? The free radical mechanism for the cracking of alkanes to
give lower alkanes, alkenes and hydrogen is given in a multi-step sequence and
explained in detail.
for GCSE/IGCSE/O level student notes see
CRACKING - a problem of supply and demand,
What is cracking? Why is it done?
- Cracking is a type of chemical
reaction in which hydrocarbon molecules, usually saturated
hydrocarbons called alkanes, are broken down into smaller molecules
by the application of heat, pressure and catalysts.
- Cracking is done to provide a
greater variety of hydrocarbon molecules from oil fractions, that
perhaps otherwise would be of little value. Apart from the fact that
oil does not contain valuable chemical stocks like alkenes, the
distribution of alkanes in oil doesn't fit the commercial profile of
hydrocarbon needs e.g. not enough lower alkane petrol or diesel fuel
- Typical example of a cracking
- octane ==> hexane +
- Cracking reactions involve complex
steps mainly involving the formation and reaction of free radicals -
h9ighly reactive species with an unpaired electron.
What is the reaction
mechanism of hydrocarbon alkanes being cracked to form alkenes,
smaller alkanes and hydrogen?
The equation is not
meant to be balanced, but just to show the variety of possible
e.g. two possible cracking reaction outcomes of propane, and all
will happen, but adjustment of temperature, pressure or catalyst
can bias the product composition towards a desired outcome.
propane ===> methane + ethene
propane ==> propene + hydrogen
See mechanism 31 below to
illustrate the idea.
hydrocarbons are heated to a high temperature (450-900oC,
with/without superheated steam) they are thermally decomposed or
'cracked' to form mainly alkanes of lower C number, alkenes of
equal or smaller C number and hydrogen.
31 - free radical thermal cracking of alkanes
Cracking occurs via free
radical reaction mechanisms and the diagram above illustrates the
free radical reactions that can result from heating even a simple
molecule like propane to a high temperature.
temperature is high enough, the kinetic energy of the particles is
sufficient to cause bond fission on collision, and this initiates a
free radical chain reaction.
initiation step when a C-C bond in an alkane molecule is split
into two alkyl free radicals
by homolytic bond fission. This means the original C-C bonding
electron pair is split between the two alkyl radicals formed.
bonds will break first in the initiation step, so the C-C
bond (bond enthalpy 348 kJmol-1) will tend to break
first as the C-H bond is stronger (bond enthalpy is 412 kJmol-1).
red dots represent the
unpaired electron on the free radical.
radicals are highly reactive species with an unpaired electron
and tend to form a new bond as soon as is possible, in this
case by ...
(2) to (6)
are chain propagation steps,
because as well as a product, a free radical is also produced to
continue the chain reaction
and lead to, in this case, a variety of other products.
(7) and (8)
are two possible chain
termination steps which remove the highly reactive alkyl free
radicals. The unpaired electrons from the two radicals 'pair up' to
form a new bond.
possible steps are shown, and not necessarily in that order all
the time, but those that are shown illustrate the possibilities
of how a wide variety of products can be formed.
You can write similar
mechanisms for any other alkane from butane upwards in carbon
keywords phrases: reaction conditions formula
intermediates organic chemistry reaction mechanisms free radicals initiation step propagation
steps termination steps CH3CH3 CH3CH2CH3 ==> CH4, CH2=CH2, CH3CH3, CH2=CHCH3, H2
CH3CH2CH3 ===> CH4 + CH2=CH2 CH3CH2CH3 ===> CH2=CHCH3 + H2 2CH3CH2. ==> CH2=CH2
and Organic Synthesis INDEX
cracking alkanes free radical mechanism
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for GCSE/IGCSE/O level notes see CRACKING
- a problem of supply and demand, other products